Hardenable polymeric materials are used in a wide variety of dental applications, including composites, filling materials, restoratives, cements, adhesives, and the like. Often, such materials shrink upon hardening. This is particularly problematic when the material is in a constrained environment, as in a dental filling or restorative, for example. Dimensional changes upon shrinkage while in a constrained environment can generate a strain within the material that is typically converted into a stress on the surrounding environment (e.g., tooth). Such forces can result in interfacial failures between the tooth and the polymeric material resulting in a physical gap and subsequent microleakage into the tooth cavity. Alternatively, such forces can lead to fractures within the tooth and/or the composite.
Generally, conventional processes of hardening polymeric dental materials involve a composite held in place on an oral surface with an adhesive and involve hardening the adhesive and then subsequently hardening the composite material. More specifically, conventional methods utilize one or more of the following steps: surface treatment of the tooth (e.g., etching, priming), application of a hardenable adhesive to the tooth surface, curing of the adhesive, placement of a composite material (e.g., restorative) on the hardened adhesive, and curing of the composite material. Such methods also typically utilize a blue light source emitting between approximately 380 nm to 520 nm to induce hardening. Photocurable dental compositions are preferably polymerized within a range of about 380 nm to 520 nm for the following reasons: 1) photoactivation utilizing UV photoinitiators or sensitizers (such as benzoin alkyl ethers, acetophenone derivatives, benzophenone and the like) that absorb light at wavelengths less than about 380 nm are generally considered to be unsafe due to the shortwavelength radiation; 2) photoinitiators or photosensitizers (such as eosin dyes, rose bengal, methylene blue and the like) that absorb light at wavelengths greater than about 520 nm are generally unsuitable due to their highly colored nature (red to blue in color) in a spectral region which is esthetically unsuitable for teeth which are generally white to slightly yellow. 3) the preferred sensitizers and initiators for dental compositions which absorb blue light between about 380 nm to 520 nm are typically pale yellow to yellow in color which provides clinically acceptable materials in terms of the esthetics of the hard tissues. Therefore, the practical limitations described have led to nearly exclusive usage of blue light. Thus, there is a need for methods of hardening dental materials, e.g., dental adhesives and dental composites, that reduce the amount of stress placed on the dental material and the surrounding environment during or after hardening.